1. Field of the Invention
The present invention relates to a surface acoustic wave filter having filtering characteristics and a balance-to-unbalance conversion function, and to a communication apparatus including such a surface acoustic wave filter.
2. Description of the Related Art
Recently, there has been significant technological progress in reducing the size and weight of cellular telephones. In order to achieve these results, in addition to reducing the number and size of components, multi-function components are being increasingly developed. In view of such a background, recent research has been actively conducted on surface acoustic wave (SAW) filters having a balance-to-unbalance conversion function, i.e., a so-called xe2x80x9cbalunxe2x80x9d function, which filters are used in the RF (radio frequency) stage of cellular telephones. Such SAW filters are used mostly for GSM (global system for mobile communications) compliant devices.
If a balanced line such as a twin lead feeder is directly coupled to an unbalanced line such as a coaxial cable, an unbalanced current flows, thereby, undesirably causing the feeder itself to operate as an antenna. A circuit called a balun is thus required for preventing the unbalanced current in order to match the balanced line to the unbalanced line.
Many patent applications pertaining to SAW filters having a balance-to-unbalance conversion function have been filed. A well-known SAW filter having a balance-to-unbalance conversion function and an input impedance and output impedance which are substantially equal is illustrated in FIG. 11.
In the SAW filter shown in FIG. 11, a comb-like electrode (also called an interdigital transducer, hereinafter referred to as an xe2x80x9cIDTxe2x80x9d) 102 is disposed on a piezoelectric substrate 100. IDTs 101 and 103 are positioned on the left and right sides of the IDT 102, respectively, in the surface acoustic wave (SAW) propagation direction.
Reflectors 104 and 105 for reflecting a surface acoustic wave in order to improve the conversion efficiency are further arranged in the SAW filter in such a manner that they sandwich the IDTs 101, 102, and 103 therebetween from the left and right. Balanced signal terminals 106 and 107 are connected to sets of electrode fingers of the center IDT 102, respectively. An unbalanced signal terminal 108 is connected to one set of electrode fingers of each of the IDTs 101 and 103.
The SAW filter of this type is called a three-IDT longitudinally coupled resonator SAW filter having a balance-to-unbalance conversion function by performing conversion between the IDTs 101, 102, and 103 using a surface acoustic wave.
In a SAW filter having a balance-to-unbalance conversion function, it is required that the transmission characteristics in the pass band between the unbalanced signal terminal 108 and each of the balanced signal terminals 106 and 107 have equal amplitude characteristics and 180xc2x0-out-of-phase characteristics.
The characteristic of the balanced signal terminals 106 and 107 having equal amplitude characteristics and the characteristic of the balanced signal terminals 106 and 107 being 180xc2x0 out of phase are referred to as xe2x80x9camplitude balancexe2x80x9d and xe2x80x9cphase balancexe2x80x9d, respectively.
The amplitude balance and the phase balance are defined as follows. If the SAW filter having a balance-to-unbalance conversion function is a three-port device consisting of, for example, a first port corresponding to an unbalanced input terminal and second and third ports corresponding to balanced output terminals, the amplitude balance=[A], where A=|20log(S21)|xe2x88x92|20log(S31)|, and the phase balance=Bxe2x88x92180, where B=|∠S21xe2x88x92∠S31|. In the above equations, S21 denotes the transfer coefficient from the first port to the second port, S31 denotes the transfer coefficient from the first port to the third port, and ∥ indicates an absolute value.
In balance between balanced signal terminals, ideally, the amplitude balance and the phase balance, as defined above, are 0 dB and 0 degree, respectively, in the pass band of a SAW filter.
However, the SAW filter in the related art shown in FIG. 11 has a problem in that balance between the balanced signal terminals 106 and 107 is reduced.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter having a balance-to-unbalance conversion function and having high balance between balanced signal terminals, and also provide a communication apparatus including such a novel SAW filter which achieves excellent high transmission characteristics.
According to a preferred embodiment of the present invention, a SAW filter includes a piezoelectric substrate, a filter unit provided on the piezoelectric substrate and having an odd number of at least three IDTs arranged along a SAW propagation direction, each IDT having a pair of electrode finger sets. Balanced signal terminals are connected to the pair of electrode finger sets of the center IDT in the IDTs, and an unbalanced signal terminal is connected to one electrode finger set of each of at least two end IDTs of the IDTs. The end IDTs are different from the center IDT, and are located on ends of the filter unit. The other electrode finger set of each of the at least two end IDTs is connected to a ground. The ground for at least one of the at least two end IDTs is electrically isolated on the piezoelectric substrate from the ground for the other end IDT.
With this configuration, the balanced signal terminals are connected to the pair of electrode finger sets of the center IDT positioned at the center of the IDTs, and the unbalanced signal terminal is connected to one electrode finger set of each of at least two end IDTs that is different from the center IDT of the IDTs, thereby implementing a balance-to-unbalance conversion function. Furthermore, the ground for at least one end IDT of the end IDTs is electrically isolated on the piezoelectric substrate from the ground for the other end IDT, thereby making it possible to improve balance between the balanced signal terminals.
In the SAW filter, a signal line which connects the unbalanced signal terminal to the end IDTs may be arranged so as to surround one of the balanced signal terminals, and a ground line may be provided between the signal line and the balanced signal terminal surrounded by the signal line. This allows the bridge capacitance inserted between the signal line and the balanced signal terminal to be reduced by the ground line, thereby making it possible to improve balance between the balanced signal terminals.
According to another preferred embodiment of the present invention, a SAW filter includes a piezoelectric substrate, a plurality of filter units provided on the piezoelectric substrate, each unit having an odd number of at least three IDTs along a SAW propagation direction, and each IDT having a pair of electrode finger sets. Balanced signal terminals are connected to the pair of electrode finger sets of a first center IDT which is located at the center of a first filter unit of the plurality of filter units, and an unbalanced signal terminal is connected to one electrode finger set of a second center IDT which is located at the center of a second filter unit, which is different from the first filter unit of the plurality of filter units. One electrode finger set of each of the end IDTs which are different from the first and second center IDTs of the plurality of filter units is connected to a ground, and the end interdigital transducers are located on ends of the first and second filter units. The ground for at least one of the end IDTs is electrically isolated on the piezoelectric substrate from the ground for the other end IDT.
With this unique configuration, the plurality of filter units increase the out-of-passband attenuation, and improve filtering characteristics. Furthermore, the balanced signal terminals are connected to the pair of electrode finger sets of the first center IDT, and the unbalanced signal terminal is connected to one electrode finger set of the second center IDT, thereby implementing a balance-to-unbalance conversion function. Moreover, the ground for at least one end IDT of the end IDTs is electrically isolated on the piezoelectric substrate from the ground for the other end IDT, thereby improving balance between the balanced signal terminals.
In the SAW filter, a package which receives the piezoelectric substrate may include a plurality of separate bonding pads for ground terminals. Preferably, the grounds for the end IDTs that sandwich the center IDT therebetween are connected to the different bonding pads so as to be electrically isolated from each other. With this configuration, the grounds for the end IDTs are separated on the package, thereby making it possible to further improve balance between the balanced signal terminals.
In the SAW filter, preferably, the IDT connected to the balanced signal terminals has an even number of electrode fingers in total. This enables the number of electrode fingers connected to one balanced signal terminal to be equal to the number of electrode fingers connected to the other balanced signal terminals, thereby further improving balance between the balanced signal terminals.
In the SAW filter, preferably, at least one SAW resonator is connected at least in series or in parallel to the filter unit. This configuration increases the out-of-passband attenuation, and can improve filtering characteristics.
In still another preferred embodiment of the present invention, a communication apparatus includes the SAW filter according to any of the above-described configurations of the various preferred embodiments of the present invention. With use of the SAW filter which can implement a balance-to-unbalance conversion function and which has improved balance, the communication apparatus can improve transmission characteristics.
Other elements, features, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.